The field of three-dimensional (3D) printing is growing explosively, due to its importance for rapid prototyping, custom manufacturing and the manufacturing of complex shapes. This printing method involves layer-by-layer printing so that the totality of the layers corresponds to the 3D object.
In spite of the growth and importance of 3D printing, the printed object is often not perfect, due to the inadequate control of the dimensions, the presence of pores, the variation of the composition, etc. Assessment of the perfection of a 3D printed object can be performed by microscopy, x-radiography, ultrasonic inspection, and eddy current inspection (which applies to metal printing and does not apply to polymer printing). Although such assessment provides valuable information on the defects present after the printing operation, it does not provide monitoring during the printing. The absence of monitoring during printing means that is not possible to correct the problem that causes the defects as the printing progresses. On the other hand, if the assessment is conducted during the printing (rather than after the printing), the defects in a particular layer can be identified as the layer is printed. With this information, the printing process can be suitably adjusted so as to avoid the formation of such defect in the subsequent layers to be printed. This renders the printing smart, as in smart manufacturing. Furthermore, the layer-by-layer assessment during the printing provides a layer-by-layer record of the quality of the printed object. Therefore, the development of methods of 3D printing monitoring is much needed.
Methods of 3D printing monitoring in the prior art involve surface profile measurement (US 2017/0056967), surface temperature measurement (US 2017/0056967), vibration sensing (EP 3170591), gas flow sensing (EP 3170593), acoustic sensing (EP 3170592), optical imaging (WO 2017039858, US 20160236414), thermal imaging (US 2015-62146871, US 20160236414), X-ray imaging (WO 2016094827), and magnetic field sensing (WO 2016094827). Other related methods include thermal infrared imaging (for observing the temperature distribution during the printing), interferometric measurement of the layer thickness, and optical analysis of the degree of curing of the resin. All of the methods mentioned above suffer from low spatial resolution and inadequate sensitivity to microscopic defects. In case of interferometric methods, an additional problem relates to the requirement that the material is transparent. Furthermore, the required placement of cameras or measurement components (sources and sensors pertaining to the optical, electromagnetic, laser, x-ray, infrared, thermal, acoustic and magnetic methods of monitoring) close to the local area of the object being printed complicates the implementation of these methods. In addition, cameras and measurement components are expensive.
In case of metal printing that involves the deposition of liquid metal droplets on a substrate, the quality or characteristics of the metal droplets are not simply related to the quality or characteristics of the solidified metal. This is because the solidified metal is formed from the deposition of a large number of droplets and how the different droplets interact and come together matter greatly to the quality and characteristics of the solidified metal. For example, the degree of bonding between the solidified droplets and the possible presence of voids between the solidified droplets affect greatly the quality and characteristics of the solidified metal in the 3D printed object. Therefore, methods to monitor the liquid metal droplets being expelled from the nozzle of the vessel holding the metal feedstock for the metal printing (US 2017/0056967) are not capable of and not applicable to the monitoring of the printed solid material during the printing.
In case of metal printing that involves the deposition of liquid metal droplets on a substrate, the efficient expulsion of the liquid metal droplets from the nozzle of the vessel holding the metal feedstock for the metal printing is important.
The capacitance, resistance and inductance are the three fundamental electronic components. The inclusion of one or more capacitors in a circuit is very common in electronics.
The capacitance describes the ability of a physical body to store an electric charge. Capacitance is measured in units known as Farads (abbreviated F). By definition of capacitance, a capacitor of 1 Farad holds a voltage of 1 Volt across the plates of the capacitor when it is charged with a current of 1 Ampere for a time period of 1 second. Instead of applying a current of 1 Ampere for 1 second, one can apply a current of 0.5 Ampere for 2 seconds and obtain the same amount of charging. The charge corresponding to this amount of charging is 1 Coulomb, since 1 Ampere is by definition equal to 1 Coulomb per second. In other words, a capacitance of 1 Farad stores 1 Coulomb with a voltage of 1 Volt across the plates of the capacitor. The charges on the two plates of the capacitor are equal in magnitude but opposite in sign. The amount of charging mentioned above corresponds to a charge of +1 Coulomb on one plate and a charge of −1 Coulomb on the other plate of the capacitor. The magnitude of the charge of an electron is 1.6×10−19 Coulomb.
Capacitances ranging from 0.1 pF (1 picofarad=1 pF=10−12 F) to 10 F can be measured using appropriate electronic meters such as LCR meters. Multimeters for voltage measurement typically allow the measurement of capacitances ranging from nanofarads (1 nanofarad=1 nF=10−9 F) to a few hundred microfarads (1 microfarad=1 μF=10−6 F). LCR meters give more accurate capacitance measurement than multimeters, in addition to allowing the measurement of a wider range of capacitance.
A capacitor is an open circuit under direct current (DC) condition, but current can pass through a capacitor under alternating current (AC) condition. The higher the frequency, the greater is the current that passes through a capacitor. The higher the voltage across the plates of a capacitor, the greater is the current that passes through the capacitor.
The capacitance of a capacitor is inversely related to the thickness of the dielectric material sandwiched by the two plates of the capacitor. On the other hand, it is proportional to the electric permittivity of this dielectric material. The electric permittivity of a material is equal to the product of the relative permittivity of the material (a dimensionless quantity) and the permittivity of free space (a universal constant equal to 8.85×10−12 F/m).
Electric polarization refers to the separation of the positive and negative charge centers in a material. It stems from the movement of charged particles (e.g., ions, electrons, etc.) in response to an applied electric field. Positively charged particles move toward the negatively charged plate of a capacitor, while negatively charged particles move toward the positively charged plate. Under an AC electric field, the polarity of each of two plates changes with time, so the charged particles (e.g., ions, electrons, etc.) of a given sign would move back and forth between the two plates as the polarity changes. The ability of the charged particles to respond to the electric field tends to decrease with increasing frequency. This is because the charged particles may not move fast enough to keep track of the changing polarity when the frequency is too high. The greater is the tendency for a material to be polarized under an electric field, the higher is the relative permittivity of the material.
Associated with the charge storage in a capacitor, as mentioned above, is electrical energy storage. This is because it takes electrical energy to separate charges of opposite sign. The electrical energy stored in a capacitor with capacitance C and voltage V (across the electrodes of the capacitor) is given by the well-known equationEnergy stored=½CV2.  (1)Therefore, both the voltage V and capacitance C must to be non-zero in order for the energy stored in the capacitor to be non-zero.
Electrohydrodynamics (EHD) refers to the study of the dynamics of electrically charged fluids. It entails the conversion of electrical energy to kinetic energy, which can be used to move a fluid. The electrical energy is given by Eq. (1). The voltage and capacitance mentioned in US 2017/0056967 are for providing the electrical energy and the associated electrostatic field that are used for the EHD-based expulsion of the liquid metal droplets out of the nozzle, as is relevant to 3D metal printing using liquid metal droplets. The voltage and capacitance in US 2017/0056967 are for providing the electrical energy and are not related to 3D printing monitoring. In particular, they are not capable of and not applicable to the monitoring of the printed solid material during the printing.
Capacitive sensing is important for touch sensing, as needed for touch screens, which are commonly used in electronic devices such as computers. Touch sensing is based on the concept that the human finger is an electrical conductor and its contact with an electrical circuit changes the capacitance of the circuit. In connection with touch screens, a large variety of electrode patterns and associated circuits have been taught (US 20170269779, US 20170024033, WO 2016037268). However, such sensing systems are not capable of and not applicable to the monitoring of 3D printing. The use of such concepts would be very expensive, due to the electrical circuit. In addition, the electrical circuit cannot withstand elevated temperatures, which are encountered in 3D metal printing.
The alternating current (AC) impedance differs from the direct current (DC) resistance in that it is a complex quantity that consists of a real part (the resistance) and an imaginary part (the capacitance and inductance, with the capacitance being more relevant to the subject of this disclosure than the inductance).
The alternating current (AC) impedance depends on the AC frequency. The variation of the electrical impedance with the frequency can be analyzed in terms of equivalent circuit models for describing the electrical behavior. The analysis typically involves the fitting of the curve in the Nyquist plot (the plot of the imaginary part of the impedance to the real part of the impedance for various frequencies). However, the equivalent circuit model obtained by the curve fitting is not unique. As a consequence of the non-uniqueness, the values of the circuit parameters (resistances and capacitances) in the circuit model are only meaningful in the context of the particular circuit model and are not generally meaningful.
The implementation of resistance measurement involves the application of electrical contacts. The electrical resistance associated with an electrical contact must be small enough, so that it does not overshadow the resistance associated with the volume of the cement-based material. Thus, the electrical contacts must be high in quality, with the electrically conductive material (typically a metal or a metal alloy) that makes up the electrical contact being in intimate contact with the cement-based material. Even if the resistance of the electrical contact is small, it may still vary as the condition (e.g., stress, strain, damage, temperature, etc.) changes. This means that both the resistance of the electrical contact and the resistance of the volume of the cement-based material change with the condition. The volume resistance is the quantity that is indicative of the condition being sensed. The variation of the contact resistance with the condition may cause the measured resistance (which includes both the contact resistance and the volume resistance) to be not indicative of the condition, thereby causing the sensing to be misleading. To alleviate this problem, four electrical contacts are used, with the outer two contacts for passing current and the inner two contacts for measuring the voltage. The resistance measured is this voltage divided by this current, and is the resistance between the two inner contacts. Because essentially no current flows through a voltage contact, there is essentially no potential (voltage) drop at each of the two voltage contacts. Therefore, the resistance obtained using four electrical contacts largely eliminates the contact resistance from the measured resistance. In contrast, the use of only two electrical contacts, with each contact serving for both current passing and voltage measurement, causes the measured resistance to include the contact resistance. In spite of the superior reliability of the method involving four electrical contacts compared to the method involving two electrical contacts, the former makes the implementation of the technique more difficult. In other words, installing four electrical contacts to measure the resistance of a segment of a cement-based structure is much more inconvenient (more labor intensive) than installing two electrical contacts.
Because the electrical impedance includes the electrical resistance (its real part), the measurement of the impedance involves the same issues as mentioned above for the measurement of the resistance. An issue pertains to the requirement that the electrical contacts are associated with low values of the contact resistance. Another issue pertains to the need for using four electrical contacts rather than two electrical contacts in order to essentially eliminate the contribution of the contact resistance to the measured resistance.
The measurement of the capacitance has its issues too. An issue pertains to the fact that an LCR meter (also known as an impedance meter) is not designed for measuring the capacitance of an electrical conductor. When an impedance meter is used for testing a conductive material, the capacitance value that it outputs can be off from the true value by a large amount (even off by orders of magnitude).
The parallel-plate capacitor geometry is commonly and classically used for measuring the capacitance of a material that is sandwiched by the two facing plates (i.e., two conductor plates commonly referred to as electrodes). The capacitance is in the direction perpendicular to the plates. Due to the small thickness of the material being tested between the two plates and the large area, the capacitance can be rather large. Thus, this variation of the parallel-plate capacitor geometry is effective for obtaining information that pertains to the capacitance. On the other hand, due to the small thickness and large area, the resistance can be rather small, though the value depends on the resistivity of the material.
In a less common variation of the parallel-plate capacitor geometry, the material being tested is positioned between the parallel proximate edge surfaces of two coplanar plates (EP 3115781). The capacitance measured is in the direction perpendicular to the two edge surfaces. This geometry tends to be associated with a small capacitance, due to the large thickness of the material being tested between the two edges (i.e., the large distance between the edges surfaces) and the small area of the capacitor (i.e., the small area of each of the edge surfaces). Thus, this variation of the parallel-plate capacitor geometry is not effective for obtaining information that pertains to the capacitance. On the other hand, due to the large thickness and small area, the resistance tends to be rather large, though this value depends on the resistivity of the material.
A parallel-plate capacitor actually involves three capacitors in series electrically, whether the electrodes are facing or coplanar. The three capacitors that are electrically in series consist of the capacitance of the sandwiched volume of the material being tested, and the capacitance of each of the two interfaces, with each interface being that between the sandwiched material and one of the two electrodes. The well-known equation for capacitors in series is1/C=1/C1+1/C2+1/C3,  (2)where C is the overall capacitance of the three capacitors (with capacitances C1, C2 and C3) in series. Hence, the measured capacitance C of the parallel-plate capacitor is given by1/C=1/Cv+2/Ci,  (3)where Cv is the capacitance of the volume of sandwiched material and Ci is the capacitance of one of the two interfaces. Thus, neglecting Ci, thereby assuming that C=Cv, can result in an incorrect determination of Cv from the measured C.
The relative electric permittivity is a material property that reflects the degree of damage in the material. The relative permittivity κ is obtained from Cv using the well-known equationCv=εoκA/l,  (4)where εo is the permittivity of free space (8.85×10−12 F/m), A is the area of the sandwich (i.e., the area of each electrode, which is the same as the area of the sandwiched material being tested), and l is the thickness of the material sandwiched by the two electrodes. Without a reliable determination of Cv, κ cannot be reliably obtained by using Eq. (4). Specifically, neglecting the term 2/Ci in Eq. (3) causes 1/Cv to be overestimated, thus causing Cv to be underestimated, and causing κ to be also underestimated.
The measurement of the impedance or capacitance requires an LCR meter (or an impedance meter). The higher the frequency, the more expensive is the meter, and the less effective is the sensing of chemical species that cannot respond to rapid changes in the polarity of the AC electric field used for the impedance or capacitance measurement. Thus, the effectiveness of a technique that operates at a low frequency is desirable.
The present invention is directed to overcoming these and other deficiencies in the art.